Two red lights, flying in a circular pattern with several repetitions over a period of at least a half hour.

Two objects, rectangular in shape, were flying in a semi-circular pattern. I was only able to distinguish them once they were going away from me. I guess looking at the back end, I could see a bright red light, and only the light. The light didn't blink, but just faded in and out over time. I couldn't see the whole flight because trees behind my house blocked the whole view. But I did see the back end of the pattern four times before I went inside my house. There were several minutes between sightings. I could also hear a low rumbling, almost like a jet engine, but it was constant, never really fading too much as the object went away, and didn't really get louder as it came close. I don't know if this was "alien" but I don't know what it is.